Transistor outline (TO) packages include a TO header and a TO cap. Optical components, such as a laser diode, may be mechanically coupled to the TO header. For example, a laser diode may be included in a chip on sub-mount assembly that is attached to the TO header. The TO header may include a header base and a header stem. For example, the chip on sub-mount assembly, which includes the laser diode, is attached to the header stem of the TO header, and the header stem is attached to the header base of the TO header. The TO cap may support a window or lens to permit the laser diode mounted to the TO header to be optically coupled to another optical component disposed external to the TO package or an optical fiber. The TO cap may be affixed to a header base of the TO header using a welding procedure, thereby providing a hermetic seal for optical components mounted to the header stem and aligning a lens or window to a centerline of the TO package and to the optical components mounted to the header stem.
Copper may be selected for the header stem to provide a low thermal resistance for the TO package, thereby permitting heat to be removed from the TO package during operation of the optical components, such as the laser diode. Steel may be selected for the header base to permit the TO cap to be welded to the header base to form the hermetic seal. However, a coefficient of thermal expansion (CTE) mismatch between the steel header base and the copper header stem results in thermal stress. The thermal stress distorts the copper header stem from a centerline of the TO package at a change of temperature of the TO package from an ambient temperature to an operating temperature. Accordingly, it may be desirable for a thermally dissipating TO package to include a CTE matched header stem, such as a copper-tungsten header stem, and header base to maintain an alignment and optical coupling of optical components inside the TO package to corresponding optical components external to the TO package at an operating temperature of the optical components inside the TO package.